I. Field of the Invention
The present invention relates to sonar equipment used by fishermen to detect the depth of the water and the depth of objects in the water such as swimming fish. More specifically, the present invention relates to such sonar equipment with a noise canceling software to eliminate signals received from other sonar devices.
II. Related Art
Prior art sonar equipment used as fish finders typically include a controller which is coupled to an oscillator, a receiver and a display. The oscillator and receiver are both coupled to a transducer. In operation, the controller controls the oscillator which, in turn, causes the transducer to transmit ultrasonic pulses throughout the water. When these pulses reach an object such as the bottom or a swimming fish, they reflect off the object as return echoes back to the transducer. When the transducer receives such a signal, it forwards the signal to the receiver which amplifies the received signal, converts it from an analog to a digital signal and forwards the digital signal to the controller. The controller is able to gauge the distance between the transducer and the object because it knows when the transducer generated the ultrasonic pulse, knows when the transducer detected the return echo and knows the speed at which ultrasonic pulses propagate through the water. The controller can then display the depth information on the display in any number of well known ways.
Systems of the type described above work well if there are no other sonar systems operating in the vicinity. If other sonars are operating in close proximity to each other, they can interfere with each other. Specifically, pulses or return echoes of such pulses generated by one can be inadvertently detected by the other, leading to confusion regarding the depth of the water or the depth of fish in the water. Thus, there is a real need for a low cost way of filtering the signals so that the equipment can distinguish between return echoes of pulses it generates from all other signals it receives from other sources.
The object of the present invention is to provide a mechanism by which a fish finder can distinguish between return echoes of pulses it generates from other signals it detects.
Another object of the present invention is to provide such a mechanism that is effective in filtering out noise and yet does not substantially increase the cost of the fish finder.
Still another object of the present invention is to provide such a filter mechanism that will not interfere with the operation of other sonar equipment operating in the vicinity.
To achieve these and other objects of the invention, the controller controls the oscillator so that the transducer produces ultrasonic pulses at a known rate different from the rate at which pulses are generated by other units in the area. As signals are detected by the transducer, they are processed by the receiver and controller. This processing includes correlation between the timing of signals received after one pulse with the timing of signals received after the next pulse. If the timing is not coherent, the controller assumes that the received signal is not an echo return of a pulse generated by the unit, but instead is noise generated by another source. The system of the present invention allows the user to select from a variety of pulse rates so that the unit can operate at a rate different than other units in the area that can produce noise, i.e. interfering signals.